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Tuesday, February 15, 2011

Your Brain on Steampunk

Going to SF/Fantasy conventions can be dangerous. Giving in to our inner fan is certainly liberating, but you have this incredible let-down when you leave the Con and return to Mundania. In the case of Dragon*Con it can be even more hazardous – to your eyes (costumes), your ears (concerts) and your mind (due to all of those ideas and concepts fizzing around in there. The following is a consequence of Dragon*Con 2009, and departs from the usual format of The LabRat's Guide to the Brain.

The Brain is Steampunk.

For years popular media has latched onto the idea that the human brain is best described as an organic form of electronic computational device. Our movies and TV are full of references to the brain as computer, from "The Terminal Man" to "The Matrix" in which it is seemingly easy to connect a brain to electronics and expand the brain into some type of super computer. Even in the medical and neuroscience fields we talk in terms of neural circuits and neural computation as if we could easily replace parts of the brain with chips and transistors and all would be right with the world.

But the brain is not electronics. It’s not even digital. Modern computers are built on binary logic. The smallest computing element is the bit – on or off, no in between. Gang enough of these tiny switches together and we get bytes, words, and the internet. Build a device that can add and subtract enough bits and we get PCs, MACs and HAL 9000. Yet, we already know that the brain does *not* operate on simply on or off. Sure, there's a function called an action potential which is all-or-nothing, but that refers just to how much voltage is produced. Action potentials can occur in singles, pairs, bursts, fast, slow, and from single neurons or billions at once. Any given neuron (single brain or nerve cell) is not exposed to simply an "on" or "off" input signal, and it doesn't produce one, either.

Unlike a computer, there is not a one-to-one relationship between the input bits and the output bits in the brain. If you trace a single memory bit through the central processing unit of a computer, you find that that bit is manipulated in isolation from all other bits, sure it can be added, subtracted, multiplied and divided, but you can track that same bit through every operation from input to output. Not so in the brain. Each neuron receives inputs from hundreds to thousands of other neurons, and in turn projects its own outputs to a similar spread of targets. In addition, those inputs can be any mix of excitatory (on) or inhibitory (off) signals. They don't even have to be "all the way on" or "all the way off", there is plenty of room for "in-between."

So the brain is not digital. Does that mean it is an analog computer? The answer is: maybe yes, maybe no. Long before ENIAC ushered in the digital age, there were mechanical analog devices in common use for astronomy, navigation and ballistics. Early electrical analog computers required dedicated wiring to connect networks or resistors and capacitors to producing variable voltage outputs. In an analog computer, voltage might represent altitude, a resistor – the wind, a capacitor – distance. It's similar in the brain – the rate at which a neuron "fires" (produces action potentials) can represent the angle at which the elbow is bent, while the difference between whether one neuron or another fires can signify whether it is the right or left elbow. Modern neuroscience has returned to the concepts of analog computing to produce better models of neurons and brain neural function.

But I digress. I actually want to convince you that neurons are not bits and transistors, but steam, clockwork and lightning. To do so I have to reiterate the basic operation of a neuron.

Neurons are cells in the body that have a special function to collect many different types of input, combine them and produce an output that can be projected to a distant part of the body or body. The first basic principle is that the outer covering (membrane) of these cells serves to separate molecules that have a charge. When salt (NaCl) is dissolved in water, it separates in positively charged sodium (Na+) and negatively charged chloride (Cl-). We call these ions, and if they can be sorted and separated, we have an electrical gradient that can be tapped to do some work. Neurons have an ingenious way to separate ions – in this case by actively pumping sodium (Na+), and positively charged potassium (K+) ions, with the Na+ on the outside and the K+ on the inside. To make this separation stick, the K+ can freely drift back into the cell through tiny pores in the neuron membrane, but sodium cannot. The corresponding negatively charged chloride (Cl-) mostly stays on the outside of cells since there's a corresponding pool of negatively charged protein inside the cell to mix with the K+. The result is that when you measure the electrical charge across a neuron membrane you find that the inside of a neuron is slightly negative with respect to its surroundings.

Keep in mind that these ions are all dissolved in water and you have "steam" part of the steampunk brain. Add in the enzymes that will actively pump Na+ out and K+ into the neuron as long as they have energy, and you have the clockwork. But what about the lightning?

Remember all that sodium pumped out of the neuron and not allowed to flow back in? The potassium can freely cross the membrane, and does, but is generally held in against high internal concentration by the overall negative charge inside the neuron. Not so for sodium. Both the high concentration and positive charge outside the neuron would push sodium into the cell if only it could pass through the neuron's membrane. Enter the sodium channel. There are pores that will allow sodium to enter a neuron, but they are normally closed. Ironically, these guardians of the neuron's electrical charge are themselves opened by small amounts of positive voltage. Opening even a single channel allows enough Na+ into a neuron to change its voltage – thus opening adjacent sodium channels spreading outward from the initial entry point and spreading the voltage ever outward. When the sodium channels are organized into a tube or pipe-like structure – such as an axon, the main component of a neuron for transmitting signals over distances – the electrical signal can quickly flash in a single direction to a destination microns to meters away. Here we have our lightning.

A combination of gates which close channels after a short interval of time (one to two milliseconds) and the sodium/potassium pump recharges our neuron just like a Tesla coil to repeated produce these lightning-like "action potentials" in as little as 5 milliseconds. At the end of the axon, internal sacs or "vesicles" hold chemicals that are when the action potential reaches the end of the line. The "neurotransmitter" chemical thus released starts the whole process in the next neuron by chemically operating still other ion-selective channel, producing "steam" and "lightning" in cell after cell.

The net result of millions and billions of Steampunk neurons acting together is even more steam, clockwork and lightning. Neurons can connect into networks that oscillate and reverberate just like the pendulum of a clock, producing a background of timing signals useful for operating muscles, producing hormonal rhythms, and separating sights and sounds in time. The analog nature of converging input and diverging output for any given neuron means that the brain itself is a mass of interconnections that defy all but the most sophisticated of wiring models. Then again, neurons and brain activity are about so much more than just the electrical signals. Neurons themselves are responsible for releasing a variety of chemicals into the blood and body which regulate temperature, attention, hunger, and thirst. Each of these means of chemicals provides yet another type of communication among neurons, and even the condition of the body feeds back to affect the operation of the brain.

So the brain is Steampunk. It's as good an analogy as any other, and better than a simple digital computer model. The entire concept captures not only the complexity, but the elegance of a totally interconnected system in which bubbling liquids, hissing steam, ticking clocks and flashing sparks all have their place. I'm sure that Agatha Heterodyne would certainly agree.